Recently, a nighttime power storage system, a home-use distributed electrical storage system based on photovoltaic power generation technology, and an electrical storage system for an electric vehicle have attracted attention, from the viewpoint of effective utilization of energy aimed at global environmental conservation and effective utilization of resources.
In these electrical storage systems, the first requirement is that an energy density of an electrical storage element used in the systems, is high. As a strong candidate of the electrical storage element having a high degree of energy density and capable of meeting other storage requirements, lithium ion batteries have been actively sought.
The second requirement is the capacitance for high output. For example, in a combination of a highly efficient engine and an electrical storage system (for example, a hybrid electric vehicle), or in combination of a fuel cell and an electrical storage system (for example, a fuel-cell electric vehicle), high output discharge characteristic is required from the electrical storage system, in order to achieve sufficient acceleration.
At present, as one type of a high output electrical storage element, an electric double layer capacitor using activated carbon as an electrode, has been developed, which exhibit high durability (in particular, a cycle characteristic and a high temperature storage characteristic), and output characteristic of about 0.5 to 1 KW/L. These electric double layer capacitors are believed to be the optimum electrical storage element in fields where the above-described high output is required; however, the energy density is only about 1 to 5 Wh/L, and output duration time limits their use in practical applications.
On the other hand, a nickel-hydrogen battery that is currently adopted in the hybrid electric vehicle attains the same level of high output as that of the electric double-layer capacitors, and has an energy density of about 160 Wh/L. However, researches have been energetically sought to further increase the energy density and the output, as well as to further improve stability at high temperature and increase durability.
As with lithium ion batteries, researches continues toward realizing the higher output.
For example, a lithium ion battery has been developed that is capable of providing a high output of over 3 kW/L, at a depth of discharge (i.e., a value indicating a state of charge in terms of percentage) of 50%; however, a lithium ion battery has been actually designed to suppress energy density equal to or less than 100 Wh/L, even though a lithium ion battery is identically characterized by the highest energy density (higher than 100 Wh/L). Durability thereof (in particular, cycle characteristic and high temperature storage characteristic) is inferior compared to that of the electric double layer capacitors. Therefore, the lithium ion battery is usable only in depth of discharge that is a narrower range than between 0 to 100%, in order to have practical durability. Therefore, usable capacitance in practice is reduced, and further research is being carried out to enhance durability.
As anther examples, a microporous membrane made of a polyolefin (for example, polyethylene, etc.), having membrane resistance equal to or less than that of a conventional microporous membrane made of a polyolefin, together with high pore content ratio (it may also be referred to as “high porosity”), i.e., high output characteristic; and a lithium ion secondary battery having the microporous membrane made of the polyolefin (refer to cited document 1 below) are proposed.
As another example, a multilayer porous membrane having a porous layer containing inorganic fillers and a resin binder on at least one-side of a polyolefin porous membrane; and a lithium ion secondary battery using the multilayer porous membrane as a separator are proposed (refer to cited documents 2 and 3 below).
Although practical application of the electrical storage element having high output density, high energy density and durability, as described above, has been required, each of the above-mentioned existing electrical storage elements have merits and demerits. Accordingly, a new electrical storage element satisfying these technological requirements has been required, and as a strong candidate thereof, development of an electrical storage element called a lithium ion capacitor has been active in recent years.
The lithium ion capacitor is one type of the electrical storage elements (i.e., non-aqueous lithium-type electrical storage elements) using a non-aqueous electrolytic solution including a lithium ion-containing electrolyte, and is the electrical storage element carrying out charge/discharge by a non-faradaic reaction based on adsorption-desorption of negative ions similar to that in the electric double layer capacitor, in a positive electrode, and by a faradaic reaction based on intercalation/deintercalation of lithium ions similar to that in the lithium ion battery, in a negative electrode.
As mentioned above, in the electric double layer capacitors that carry out charge/discharge by an non-faradaic reaction in both of the positive electrode and the negative electrode, the output characteristic is superior, but energy density is low. On the other hand, in a lithium ion battery, which is a secondary battery that carries out charge/discharge by a faradaic reaction in both of the positive electrode and the negative electrode, the energy density is superior, but the output characteristic is inferior. A lithium ion capacitor is a novel electrical storage element which can achieve both superior output characteristic and high energy density, by carrying out charge/discharge based on the non-faradaic reaction in the positive electrode and based on the faradaic reaction in the negative electrode.
Applications using the lithium ion capacitor include electricity storage for railways, construction equipment, and automobiles. It is necessary to have superior temperature characteristic, because operating environment is severe in these applications. Specifically, the temperature characteristics mean a high input/output characteristic at low temperature, or a high cycle life characteristic at high temperature.